1. Field of the Invention
The present invention relates to an image pickup apparatus used for, for example, a single-lens reflex camera.
2. Description of the Related Art
Widely used single-lens reflex cameras include movable mirrors and have a structure in which the mirrors are moved away from the image capturing optical path just before a photographing operation and the mirrors return to the optical path immediately after the photographing operation to guide subject light to a viewfinder optical system. As a result, a viewfinder image disappears during the photographing operation and the subject is not viewed at the moment of the photographing operation. In particular, when photographs of sports, racing, animals, and the like are taken, the motion of the subject is fast, whereby it is quite difficult to take photographs with the subject being followed during continuous shooting operations. Consequently, a critical moment is sometimes missed.
On the other hand, there have been proposed single-lens reflex cameras having a function of splitting light beams with a fixed mirror (e.g., refer to Japanese Unexamined Patent Application Publication Nos. 3-109504 (Patent Document 1), 2006-350253 (Patent Document 2), and 8-254751 (Patent Document 3)). This type of single-lens reflex camera includes a light-beam splitting mirror (also referred to as a pellicle mirror) that is a fixed mirror which guides light beams to a viewfinder system. This mirror splits photographing light in a direction toward a viewfinder system and in a direction toward a film surface at all times, which allows a user to observe the state of a subject through a viewfinder during the photographing operation.
Patent Document 1 discloses a film design of a light-beam splitting mirror used for a single-lens reflex camera. In Patent Document 1, the intensities of transmitted light (light guided to an image sensor) and reflected light (light guided to the viewfinder side) when a light-beam splitting mirror is disposed at 45 degrees are each set to be 50%. In this case, since the quantity of light provided to a film decreases, exposure time is increased and the range of light of a flash is shortened. To solve such problems, it is necessary to provide the light-beam splitting mirror with a movement mechanism (refer to Patent Document 3) included in widely used mirrors when photographs are taken using a flash.
Furthermore, Japanese Patent No. 2801217 (Patent Document 4) discloses a system in which an autofocus detecting device is disposed at a lower position in a camera and light transmitted through a light-beam splitting mirror is again guided to the autofocus detecting device using a movable mirror. In this system, manufacturing cost may be increased due to the complicated mechanism of the system and dust may be produced due to the presence of a movable member on the inner side of the fixed mirror.